In order to systematize the collection of multifold data, it is desirable to lay out a set of grid points as shown by the indexing scheme in FIG. 1. With this well-known arrangement, a system of standard static correction equations can be conveniently characterized as follows: EQU T.sub.i,r =I.sub.i +R.sub.r +C.sub.d +(X.sub.i,r.sup.2 M.sub.d) EQU d=i+r
wherein i is the source location index, r is the group index, d is the depth index, T.sub.i,r is the observed two-way time for the seismic energy to travel the particular ray path from source point, i, to group, r, I.sub.i is the initiation static-error term, R.sub.r is the receiver static error term, C.sub.d is the true two-way time which is desired for mapping, and the product X.sub.i,r.sup.2 M.sub.d is the normal moveout correction term.
For the data acquisition arrangement shown, there would be 196 such equations (i.e., 14 shot points.times.14 receiver locations). With this arrangement, separate shots are initiated at each of grid locations 1 through 14 while seismic recordings are being made at receiver locations 1 through 14. Fourteen-fold data is thus produced for depth index 15, with the fold (multiplicity) tapering to a value 1 at each end of the seismic line as shown in FIG. 2.
In certain seismic exploration areas where the static correction problem is not especially difficult, it is a common practice for seismic crews to attempt to increase their efficiency by regularly skipping index locations. This practice, as depicted schematically in FIG. 3, results in the production of "partial" multifold seismic data. In this Figure, fully one-half of the possible source locations have been skipped in a regular alternating manner, thus reducing the fold from 14 to seven, as depicted in FIG. 4. This approach commonly results in significant savings of time and money without seriously adversely affecting the signal-to-noise ratio and the quality of static correction of the recorded data. There often is, however, an undesirable "zig-zag" pattern which appears in the resulting seismic section when the prior art "partial"multifold approach such as depicted in FIG. 3 is used. FIG. 7 shows a seismic section having the above-mentioned zig-zag pattern superimposed thereon, as indicated by the triangles and circles. It will be appreciated by the reader that the typical zig-zag patterns shown in FIG. 7 clearly detract from the interpretability and accuracy of the displayed seismic section. This problem has long plagued practicing geophysicists.
It is therefore an object of the instant invention to provide a method for seismic exploration using a new "partial" multifold data collection technique which substantially eliminates the production of zig-zag patterns on the corresponding seismic section displays. The method of the instant invention results from the discovery by the Applicant of the reason for the zig-zag patterns of the prior art, as well as a practical field technique for eliminating the same.